The present invention relates to data storage systems. More specifically, the present invention relates to data storage systems using spin tunnel magnetoresistive read heads.
A magnetoresistive (MR) element exhibits a change in electrical resistance as a function of external magnetic field. This property allows MR elements to be used as magnetic field sensors, read heads in magnetic storage systems, and magnetic random-access-memories. In storage systems, the read head is typically merged with a writer head. The writer writes encoded information to the magnetic storage medium, which is usually a disk coated with hard magnetic films. In a read mode, a magnetic bit on the disk modulates the resistance of the MR element as the bit passes below the read head. The change in resistance can be detected by passing a sensing current through the MR element and measuring the voltage across the MR element. The resultant signal can be used to recover data from the magnetic storage medium. Depending on the structure of a device, the MR effect can fall in to different categories, namely, anisotropic MR (AIMR), giant MR (GMR), tunneling MR (TMR), and colossal MR (CMR).
Many hard disc read heads currently in production utilize an AMR sensor. The essential structure consists of a stripe of soft magnetic material, usually an alloy of Ni, Fe and/or Co. For areal densities beyond about 10 Gbit/inch2, AMR heads give way to GMR heads due to lack of signal.
The GMR device favored by the data storage industry is the spin valve. It consists of a free ferromagnetic layer which rotates with the external field, a conductive spacer, and a pinned ferromagnetic layer which has a magnetization fixed along one direction. The electrical resistance of a spin valve is a function of the angle between the magnetization in the free layer and the pinned layer. A GMR sensor is the most resistive when the two layers are magnetized in anti-parallel directions, and is the most conductive when they are parallel. Most companies have completed the transition from making AMR heads to making GMR heads. The technology can possibly work for areal densities up to 100 G bit/inch2, beyond which point the sensitivity again becomes an issue.
One possible solution is to use TMR junctions, which can give two to three times more signal. In addition, TMR junctions offer more room for engineering design, as the TMR effect is less sensitive to the structure of the element than GMR. In particular, the magnetoresistance is usually not related to the junction area, junction resistance, and film thickness. TMR read heads have been disclosed, for example, in the following United States Patents which are herein incorporated by reference in their entirety: U.S. Pat. No. 5,390,061 assigned to Hitachi, Ltd; U.S. Pat. Nos. 5,729,410, 5,898,547, 5,898,548, and 5,901,018 all assigned to IBM.
A TMR junction is very similar to a spin valve in the sense that it also consists of a free layer, a spacer, and a pinned layer. The magnetoresistance rises from the angular difference between the magnetization in the two magnetic layers in a way entirely analogous to a spin valve. A major difference between a TMR junction and a spin valve is that the spacer in a TMR junction is made of an insulator, typically aluminum oxide, instead of a conductor. Moreover, in conventional TMR sensors the electrical current is perpendicular to the plane of the films as opposed to in the plane for GMR sensors. Consequently, one must attach a top and a bottom electrode to the junction stack in order to measure the electrical property.
Spin dependent tunneling effect has been proposed for higher areal density recording above 40 Gbit/inch2. The high TMR ratio of the junctions offers much better sensitivity in the transducer as discussed above. To achieve higher areal density, it is essential to use a smaller shield-to-shield spacing. A 10 Gbit/inch2 head requires about 1200 xc3x85 shield-to-shield spacing. For a 40 Gbit/inch2 head, this value becomes less than 900 xc3x85. Assuming an optimistic gap thickness of 600 xc3x85, this means the TMR stack has to be less than 300 xc3x85 thick. This can be unrealistic for both GMR stacks and conventional TMR stacks due to the minimum thicknesses of the pinning layer ( greater than 25 nm for NiMn). In the case of TMR, the situation can be worse due to the fact that contact pads of more than 10 nm of metal film may be required to guarantee uniform current distribution within the junction, thus making a TMR stack thicker than an equivalent GMR stack.
A spin tunnel junction magnetoresistive head in accordance with the invention includes a pinned ferromagnetic layer, a free ferromagnetic layer and a spin tunnel barrier material positioned relative to the pinned and free ferromagnetic layers such that current flowing through the free ferromagnetic layer is in the plane of the free ferromagnetic layer. The spin tunnel barrier material forms first and second edge junctions. Using the edge junctions, the free ferromagnetic material, the pinned ferromagnetic and the edge junctions can all be formed at least partially in plane with each other, reducing shield-to-shield spacing for the head.